1. Field of the Invention
The present invention relates to a steel plate to be used for steel structures (such as bridges and towers) which present difficulties in routine maintenance work such as repainting and also to a manufacturing method thereof. More particularly, the present invention relates to a steel plate to be used with a painted film in coastland or cold district where steel structures are subjected to salt damage by airborne salt or deicing salt (as an antifreezing agent scattered on the road) and also to a manufacturing method thereof.
2. Description on the Related Art
There are two kinds of corrosion-resistant steels specified in the Japanese Industrial Standards (JIS). They are corrosion-resistant hot-rolled steel for welded structures (designated as SMA, JIS G-3114) and highly corrosion-resistant rolled steel (designated as SPA, JIS G-3125). These steels contain Cr, Cu, Ni, P, etc. in an adequate amount. Such corrosion-resistant steels are also disclosed in Japanese patents mentioned later. Corrosion-resistant steels form a dense and excellently adhesive layer of stable rust thereon which protects them from corrosion. They have been widely used in inland areas.
Unfortunately, corrosion-resistant steels need a long time of 10 years or more until they form a layer of stable rust. Practically, they pose a problem of initial corrosion and rust-laden water. This is true particularly in Japan where the climate is warm and humid. Rust stabilization is common practice to prevent corrosion-resistant steels from posing landscape or environment with rust-laden water until they form stable rust when they are used without a painted film. This practice, however, merely avoids rust-laden water and hinders the formation of compact rust layer when steels are used in a salt-polluted environment.
Several means have been proposed to address the above-mentioned problems involved in corrosion-resistant steels. For example, resin painting on the surface of corrosion-resistant steel, which is intended to promote the formation of stable rust while isolating steel surface from its environment, is disclosed in Japanese Patent Publication Nos. 22530/1978, 33991/1981, 39915/1983, 17833/1983, and 21273/1994, and Japanese Patent Laid-open No. 133480/1990. A surface treating solution to promote the formation of stable rust, which contains Fe3O4 of scaly crystal structure, phosphoric acid, and butyral resin dissolved in a solvent, is disclosed in Japanese Patent Laid-open No. 133480/1990. A method of surface treatment for rust stabilization, which consists of applying a painting solution composed of more than one compound of P, Cu, Cr, N, Si, and Mo, Fe2O3+Fe3O4, phosphoric acid, a bisphenol epoxy resin, and auxiliaries dissolved in a solvent, is disclosed in Japanese Patent Publication No. 21273/1994. The above-mentioned means, however, neither improve the corrosion-resistant steels themselves nor promote the formation of stable rust satisfactorily. In other words, a resin painted film usually has minute defects at which the film effect is not produced. Such defects cause corrosion to take place in the interface between the painted film and base metal, with the result that the painted film exfoliate before the stable rust layer is formed. Therefore, the use of corrosion-resistant steel is limited in the salt-polluted environment.
In the meantime, an important subject in the world of bridge is to save maintenance cost for repainting as well as construction cost. The latter object is achieved by reducing the number of main girders, adopting rationalized girders, reducing the frequencies of site welding, and reducing maintenance management. This stimulates a demand for steel with large thickness and high strength capable of welding with a large amount of heat input which obviates preheating to prevent cold cracking at the time of welding.